Thermal and physiological constraints on energy assimilation in a widespread lizard (Sceloporus undulatus)

Thermal constraint on energy assimilation is an important source of life history variation in geographically widespread ectotherms such as the eastern fence lizard (Sceloporus undulatus). Fence lizards in southern populations grow faster and produce more offspring per year than do those in northern populations. Biophysical models indicate that this difference in production is the result of thermal constraints on energy assimilation, but they do not exclude intraspecific variation in behavior or physiology. I quantified both thermoregulatory behavior and the thermal sensitivity of metabolizable energy intake (MEI) in lizards from New Jersey (NJ) and South Carolina (SC) populations of Sceloporus undulatus. In the laboratory, I conducted feeding trials to estimate MEI at body temperatures experienced by field-active lizards (20°, 30°, 33°, and 36°C). I also measured preferred body temperature (T_p) of lizards in a thermal gradient. In the field, I estimated the accuracy of thermoregulation by lizards. Both NJ and SC lizards exhibited a maximal MEI at their T_p (33°C), but lizards from SC had a significantly higher MEI at this temperature than lizards from NJ. Although lizards in both populations thermoregulated within 2°C of T_p, lizards in SC could maintain T_p for a longer duration on a daily and annual basis. Therefore, lizards in SC could assimilate more energy because they had a higher maximal MEI during activity and were active for longer durations than lizards in NJ. Geographic variation in the life history of S. undulatus may be caused by differentiation of physiology between populations, as well as by differences in the thermal environments of populations.